Automobile exterior parts such as emblems, front grilles (radiator grilles), and door handles of automobiles are subjected to chrome plating. The chrome plating improves aesthetic appearance, increases surface hardness to prevent scratches, and furthermore provides corrosion resistance to avoid rust.
Conventionally, plated parts sequentially coated with a substantially non-sulfur semi-bright nickel plating layer, a bright nickel plating layer, an eutectoid nickel plating layer (a distributed strike nickel plating layer), and a chrome plating film on a substrate have been disclosed as chrome-plated parts (see Patent Citations 1 to 3). In these conventional arts, it has been disclosed that an electrochemical potential of the nickel plating layer is controlled within a predetermined range so as to prevent detachment of the chrome plating layer.
Patent Citation 1: Japanese Patent Unexamined Publication No. H05-287579
Patent Citation 2: Japanese Patent Unexamined Publication No. H06-146069
Patent Citation 3: Japanese Patent Unexamined Publication No. H05-171468
Recently, corrosion cases in a specific circumstance have been recognized. Specifically, one case is that a chrome plating layer as a surface is corroded prior to a nickel plating layer as a base, which causes aesthetic appearance to get worse, and another case is that gas, which makes plated parts swollen, is generated by severe corrosion of a nickel plating layer as a base. Such cases have highly occurred to decorative chrome-plated parts of various types of automobiles, especially, such as front grilles, emblems and door handles. A snow-melting agent used for avoiding roads being frozen and hygroscopic salt (such as calcium chloride, magnesium chloride and sodium chloride) used for avoiding road dust being dispersed adhere to these parts with an adsorptive material such as mud. The concentration of salt (chloride ion) on the parts to which the snow-melting agent is adhered increases due to water evaporation.
In such a case of being covered with chloride ion at high concentration, and under environmental condition with hot and cold cycle of a heated motor garage and an outdoor location of which temperature drops to below freezing, the severe corrosion has been caused.
For purpose of enhancing the corrosion resistance in such a specific circumstance, a method for forming a passive film on the chrome plating layer using an oxidizing agent is disclosed (see Patent Citations 4 to 7).
Patent Citation 4: Japanese Patent Unexamined Publication No. 2005-232529
Patent Citation 5: Japanese Patent Unexamined Publication No. 2007-056282
Patent Citation 6: Japanese Patent Unexamined Publication No. 2007-275750
Patent Citation 7: Japanese Patent Unexamined Publication No. 2008-050656